In medium skillet, cook sausage until brown all the way through, crumbling it as it cooks. Remove to paper towel to drain off excess grease. Discard all but a tablespoon or two of the grease remaining in skillet. Cook onion until soft and translucent. Add sausage to skillet and mix together. Set aside.
Turn on oven broiler. Place poblano peppers on cookie sheet and put under broiler for about 5 minutes or until black and blistered. Let cool, then dice.
Turn oven to bake and 400 degrees.
Divide the cheese into thirds and the sausage in half. In an 8 or 9-inch cast iron skillet, layer 1/3 of the cheese, 1/2 the sausage. Repeat and top with the remaining cheese.
Bake in oven 4-5 minutes or until cheese is melted and starting to bubble. Keep an eye on it! Remove from oven before the cheese starts to brown.
Immediately after removing from oven, fold in 1/4 cup milk and stir gently until cheese is blended and smooth. You want to do this quickly. Top with chopped peppers, diced tomatoes and cilantro. Serve immediately with tortilla chips for dipping.